The present invention relates to synchronization schemes in telephone switching systems and more particularly, to a method and apparatus for synchronizing digital data signals received from telephone stations coupled to a common equipment unit (CEU), such as a key service unit (KSU), of a telephone switching system, to the time base of the CEU. Such synchronization is necessary because of the variable delays encountered in transmitting data to and from remotely located telephone stations over the station transmission lines.
In order to synchronize data from digital telephones relative to the timing in the CEU or KSU, it is necessary to provide a means for compensating for the external delays that are encountered. A phone that can be located a variable distance from the KSU will encounter delays caused by the propagation delay of the transmission media. For standard twisted pair, the propagation delays are on the order of 2 .mu.secs. per kilofoot of cable. Thus, if a phone is located at 2 kilofeet from the KSU, a loop delay of 4 microseconds will be encountered (2 .mu.secs. to the phone and 2 .mu.secs. back). In order to compensate for these delays, a method and apparatus for determining the amount of delay and for compensating for this delay is required.